Bernice Polansky's mysterious symptoms began creeping through her body in 1989, four years after she started working at Southwest Airlines' 24-hour San Antonio, Texas, reservations center, an amphitheater-like building housing 600 agents. First came the headaches--every day, two hours after she arrived at work. She noticed other agents bringing aspirin to work. Anyone who ran out could go down to the central-console area, where supervisors were dispensing aspirin from large bottles. Polansky joined the aspirin poppers.

Then came the sinus infections, muscle pain, nausea, dizziness and fatigue--"a whole body weakness." Others complained of weakness too, though no one seemed to...